My central Air Conditioner Unit is making a WEIRD sound!?

I live in an apartment that was built in the mid-seventies. Needless to say, everything is pretty old. The air conditioner, the dishwasher - it all works, but it's old by today's standards. My question is, my central air makes this HORRIBLE sound when it's running. I'm really not trying to be funny here, but the closest thing it sounds like is Chewbacca from Star Wars. It's this really high-pitched vibration. It only does it when it's kicked on, obviously. We can't figure out what is making this sound. The sound is actually coming from the unit which is stored inside a closet in my kitchenette. The unit is cemented shut so we can't figure out what's wrong. Its a Singer BCA 22000, 1976 model. I can find nothing about it on the Internet. ANY help is appreciated, because the sound is driving us BATTY!